An IC tag also called a radio frequency identification (RFID) tag has been known. In recent years, there has been a need to increase a communication distance to be longer than a conventional one. Accordingly, a method of communication between an IC tag and a reader writer has been changed from an electromagnetic induction method to a radio communication method. A communication frequency has also been changed from a conventional high-frequency (HF) band to an ultrahigh-frequency (UHF) band capable of performing a radio communication, and a half-wave dipole antenna has been frequently used as an IC tag antenna.
A half-wave dipole antenna is a dipole antenna, in which an antenna length that is an effective length of a radiation part of an antenna (described as an antenna radiation part in the present application) is made to be about ½ of a communication wavelength (refer to Patent Literatures 1 and 2, for example).
FIG. 9 shows a basic configuration of a half-wave dipole IC tag antenna. A half-wave dipole antenna shown in the drawing comprises an IC chip mounting part 101 having an IC chip mounting terminal 103 to mount an IC chip (not shown), an impedance adjustment part 102 to adjust impedance of an antenna, and a pair of antenna radiation parts 11.
In the half-wave dipole antenna shown in FIG. 9, a central branch path 10 is composed of an IC chip mounting part 101 and an impedance adjustment part 102, and is divided at two points at which the antenna radiation parts 11 are connected. Of the central branch path 10, a part having an IC chip mounting terminal 103 is an IC chip mounting part 101, and the other part is an impedance adjustment part 102. This configuration constitutes, in a high-frequency circuit, a circuit virtually equivalent to a circuit shown in FIG. 10.
In an IC tag, generally, an IC chip has no power supply, and operates on the power received by an antenna. Therefore, the antenna radiation part 11 is required to supply power to an IC chip by achieving impedance matching with a not-shown IC chip to be used by connecting to an IC chip mounting terminal 103.